Pipe Thread vs Aeronautical National Pipe Thread
NPT vs ANPT
The requirements for NPT (National Pipe Taper) screw threads are spelled out
in ANSI B1.20.1
The requirements for ANPT (Aeronautical National Pipe Taper) screw threads are
spelled out in ANSI SAE AS71051
The NPT and ANPT product screw threads in the sizes up to and including
2 inches are identical. Beginning at 2-1/2 inches NPT and ANPT product screw threads are NOT identical. Beginning at 2-1/2 inches the wrenching
length of the coupling is different between NPT and ANPT product
screw threads. NPT has 2 threads and ANPT has 3 threads. There are major
differences in the gages.
NPT and ANPT screw threads are designed to screw together. In most cases
there will be no interference between the root and crest of the
threaded at assembly. The tapered screw thread is designed to be assembled with
some form of sealant to assure a leak free joint.
NPTF screw threads will screw together with both NPT and ANPT screw threads
and should have no noticeable assembly problem. There will most
likely be an interference fit between the root and crest on either
the major or minor diameter of the screw thread, depending on which part
is NPTF. To accomplish a seal on the joint, a sealant will be required.
Both NPT and ANPT screw threads have a requirement for an L1 gage. The
gage is called L1 because it relates to the L1 dimension in the
screw thread specification. Even though the L1 requirement is identical
for both NPT and ANPT product screw threads, the tapered pipe screw thread gages are different at either the
major diameter or the minor diameter. From my quick survey of the
two standards I found: the ANPT-L1 tapered pipe screw thread plug gage major diameter was larger
than the NPT major diameter; the ANPT-L1 tapered pipe screw thread ring gage minor diameter was
larger than the NPT minor diameter. All other L1 gage parameters
are identical between the NPT-L1 and ANPT-L1 gages. Technically;
the NPT-L1 and the ANPT-L1 gages are not fully interchangeable.
The NPT screw thread standard only defines a L1 ring
gage for the male thread and a L1 plug
gage for the female thread. Both gages measure the hand tight
L1 length of the screw thread.
There are no such NPT tapered pipe screw thread gages: L2; L3 or 6-Step Crest Check. If
you have NPT tapered pipe screw thread gages marked: L2; L3 or 6-Step Crest Check, they are
really ANPT gages which have been inappropriately remarked to meet
a customer's demand. The gage maker should have educated the customer
instead of supplying the incorrectly marked taper pipe screw thread gages. If these inappropriately marked taper pipe screw thread gages were sent
to a calibration laboratory, the calibration laboratory should return the gages
without calibration because there is no standard which defines the
If your NPT product design requirements indicate additional measurements
must be taken, consider changing the product requirement to ANPT
per ANSI SAE AS71051. Because the majority of the taper pipe screw thread applications will be 2 inches or less; and because the NPT and ANPT product screw threads in the sizes
up to and including 2 inches are identical; this should present no problems with the end product. If your product is over 2-1/2 inches
some design consideration should be conducted.
ANPT requires: both L1 and L2 ring
gages for the male thread; both L1 and L3 plug
gages for the female thread; and Crest Check (frequently called
6-step) gages for both the internal or external thread.
The data provided is accurate to the best of my knowledge. Please
use it at your own risk. Original Posting: 3/21/2008 Last Revised: 5/25/2011 Refer comments/corrections to firstname.lastname@example.org.